ARM: sa1100: normalize clk API

sa1100 provides its own variant of the clk API rather than using the
generic COMMON_CLK API. This generally works, but it causes some link
errors with drivers using the clk_set_rate, clk_get_parent, clk_set_parent
or clk_round_rate functions when a platform lacks those interfaces.

This adds trivial stub implementations for each of them, based on
the behavior of the COMMON_CLK implementation:

- set_rate() and set_parent() report success without doing anything
- round_rate() returns the clk rate
- get_parent() returns NULL.

This adds the minimal bloat and should do the right thing for
the simple clock hardware in this SoC.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>